Suppression of echoes and singing in four-wire circuits



Nov. 6, 1928. 1,690,182

H. c. SILENT SUPPRESSION OF ECHOES AND SINGING IN FOUR-WIRE CIRCUITS Filed Sept; 8, 1927 Hm] bid Loiand 4dr: lfizeor'fiwmlambllhlc l 5 D [ID s uneaq 1 I 1 wal y: 2

+ M 2am I illl imzzcm zm ofall batterL'eS. am Link.

' Z .z]o 1 I Y Z IN V EN TOR.

1Z6. sum;

A TTORNEY Patented Nov. 6, 1928.

UNITED STATESPATENT OFFICE.

mom) 0. SILENT, or LAnonmoNr, New YORK, ASSIGNOR 'LO AMERICAN rnnnrnonn AND TELEGRAPH eoMrAnY, A CORPORATION or new YORK.

SUPPRESSION F ECI-IOES AND SINGING IN FOUR-WIRE CIRCUITS.

Application filed September 8, 1927. Serial in). 218,258.

This invention relates to two-way communication system, and more particularly to two-way telephone systems which include, between two-wire lines, a four-wire circuit having one path adapted for transmission in one direction and another path adapted for transmission in the opposite direction and resides in improved methods of and means for suppressing echoes and singing in such in systems.

The invention is applicable to systems which include, between two-wire lines, a

, tour-wire circuit containing a radio link. It

is also applicable to all-wire twoway systerns in which the four-wire circuit covers the entire distance between two two-wire lines without the intervention of a radio link.

In order to simplify the discussion of a specific embodiment of the invention, it is proposed to disclosespecifically only one portion of a two-way telephone system to iflvhich a set of devices for suppressing echoes and singing is connected, and this section of two-wire line nd one end of the radio link.

In a telepiione system of this class, in which the two" paths of the'four-wire circuit are adapted for transmission in opposite directions, it is, under certain conditions, a re quirement, if the best possible results are to be secured, that one path of the four-Wire circuit be normally blocked and consequently normally disabled. This arrangement serves to prevent singing in the circuit in its normal condition. In addition, in association with each transmission path of the four-wire c ircuit there must be provided means responsive to waves in this path for blocking the transmission over the other path or for preventing the removal of a shortcircuit already on the path, in order to secure the suppression of echoes returning from .the two-wire line over that other path. The arrangement of the four-wire circuit which is ordinarily prefthe complete system which lies between one erable is that in which the outgoing path that is, the one adapted for transmission to the radio link-is normally blocked, while the path adapted for reception from the radio link is normally cleared. With this arrangement it is, of course, necessary to pro: vide means' responsive to voice waves in the outgoing path for clearing this transmission pat 1.

In the arrangement of four-wire circuits which has been found comparatively successful in all-wire systems, an amplifier-detector device is associated with each path of the c1rcuit. Voice waves in the transmitting (oroutgoing) .pathenter the associated amplifier-detector, and the output of the detector causes the operation of a relay or relays to place a short circuit on, or otherwise block, the receiving path, and also to clear the transmitting path. Voice waves in the receiving path, provided this path is not blocked at the time, enter the associated amplifier-detector, and the output causes'the operation of a relay or relays to block the transmitting path or to prelvlent the removal of a. short-circuit on that pat One of the chief objects of the applicants improved arrangement for controlling the paths of a four-wire circuit is to provide protection against echoes which is directly proportional in degree to the strength of the "transmission which causes the echoes.

Another object of the invention is to give a positive hangover to the protection ailorded by the voice operated devices which may be adjusted as to time of duration.

A. further object/of the invention is to provide protection against echoes which may be adjusted either to be practically constant during the hangover period or to decrease graduall to a value just. suilicient to take care oft e decreased amplitude of the echoes.

Furthermore, one of the chief. objects of the invention is to provide for protection against eehoes'which is abruptly removed at the end of the hangover period.

In attaining the objects stated above, the applicant, in general, employs a detector associated with both transmission channels of the founwire circuit for controlling the transmission path clearing and blocking switches, impresses a negative voltage on the grid of this detector in response to waves in the path ada ted for transmission from the radio link, t is voltage being suficient to BIB prevent operation or the detector by echoes returning in the opposite path, automatically stores a quantity of energy to maintain this negative voltage on the detector grid, and provides for the abrupt discharge of the maintaining energy at the termination of the hangover period, the length of this period being readily adjustable.

By virtue of the arrangement briefly described ahove and to be more fully described and discussed hereinafter, the protection against operation of the controlling detector by echoes in the outgoing path is directly proportional to the strength of the trans: mission in the incoming path. Furthermore, the length of the hangover period is quite independent of the amplitude of the waves flowing in the incoming path of the fourwire circuit toward the two-wire line.

These and other advantages of the invention will be more clearly understood when the following detailed description of one desirable embodiment of the invention is read with reference to the accompanying drawing.

Figure l of the drawing shows diagrammatically a four-wire circuit connected at one end directly to a two-wire line and at the other end, as indicated, to a two-wire line either directly or through a radio link, and having associated with it the improved apparatus for controlling the clearing and blocking of the onc-way paths of the circuit.

Fig. 2 shows a curve diagram indicating the voltages which control the hangover in the case of a wave of relatively great amplitude'in the incoming path of thefour-wire circuit.

Fig. 3 shows in like manner the voltages which determine the hangover in the case of a wave of relatively small amplitude in the incoming path.

With reference first to the details of Fig. 1, there are shown a four-wire circuit connected at its left end to a two-wire line, through the usual hybrid'coil, and a network approximately balancing the two-wire line. As indicated in this figure of the drawing, the four-wire circuit is connected at its right end to a radio link, for instance. The lower path of the circuit is adapted for transmissionfrom the two-wire line to the radio link, and the upper path is adapted for transmission from the radio link to the two-wire line. In each path, there is the usual one-way repeater.

When speech comes from the two-wire line, the voice currents pass over the lower path of the four-wire circuit which is normally blocked by a short circuit under the control of a relay S The voice currents in this outgoing path enter the tuned amplifier associated with the path and act on a detector D to operate a switching device specifically shown as a relay S. When this relay operates, it closes a circuit through a battery and the windings of relay S and a relay S, which controls the upper transmission path. This upper path, adapted for transmission from the radio link, is normally cleared, but a short circuit may be placed on it by the operation of relay S A delay circuit, positioned in the outgoing path as shown, delays the voice currents in the transmitting path until the detector D has had time to eti ect the switching operations discussed above. Thus, when-voice currents come over the two-wire line, the normally blocked outgoing path is cleared and the normally cleared incoming path is blocked; thus the talker over the two wire line is automatically given control of the circuit.

There is, however, danger that voice currents, or currents due to static or other disturbing causes, may enter the upper path of the four-wire circuit from the radio link and be reflected back from the two-wire line over the lower or outgoing transmission path of the four-wire circuit, since the balance of the twowire line by the associated network is not perfect. It will readily be understood that if these e choes in the outgoing path are permitted to operate the detector D and the switches controlled thereby, transmission over the telephone system will be seriously interrupted.

The applicant associates two tuned amplifiers with the upper or incoming transmission path and interposes between these amplifiers and the detector D which controls the switches, an arrangement of vacuum tubes and related circuits as described below. It is believed that the apparatus involved will be most clearly understood when discussed in connection with the operation of the circuits in response to waves in the upper transmission path.

A portion of the speech or other current coming into the upper path of the four-wire circuit from the radio link enters the tuned amplifiers. The current entering the upper one of the amplifiers associated with the incoming path causes a detector D to charge a condenser C connected across the filamentplate circuit and impresses a negative biasing voltage on the grid of the detector D which is added to a small normal biasing voltage from the grid battery. This negative voltage impressed on the grid of D. it will be readily understood, is directly proportional in its value to the strength of the transmission in the upper path and accordingly is directly proportional to the strength of any echo currents caused by the incoming currents and reflected from the two-wire line over the lower path. Thus, there is automatically placed on the grid of D a negative volta e suflicient to prevent the operation of D, hy the returning echoes.

Associated with the condenser C, in the output of D, is a resistance R This resistance is a high resistance and serves to maintain on the grid of D a negative voltage for the biasing of the grid during idle periods. If the value of this resistance were infinite, the charge on C, would continue indefinitely or until released by some auxiliary action. The release of the charge on C, and the con sequent removal of the protection bias on the detector D is the function of the vacuum tube V, and the lower set of vacuum tubes and associated circuits (with reference to F ig. 1). The current entering the lower of the tuned amplifiers causes a charge to be placed on a condenser C placing a potential across the associated resistance R This lower circuit is associated with the vacuum tube V,, upon the grid of which a normal negative voltage is impressed by the'battery B. The charge on the condenser C makes the rid of V, quite negative with respect to the filament of V,, and consequently, no current can flow in the plate circuit of V The value of the resistance R, is so chosen that the charge on the condenser C leaks oil graduall Consequently, the negative biasing vo tage on the grid of V is radually reduced. When the charge on 5 has leaked ofi sutficiently, so that the grid of V, is practically at the normal biasing voltagefrom the battery B, the filament-plate circuit of the tube denser C, is immediately discharged through that-circuit. Thus, it will be seen that the negative voltage applied to the grid of the detector D to prevent operation of the detector and the controlled switches by echoes in the lower path is maintained through a period of time determined by the action of the circuit C R -that is, by the values of the condenser and the resistance. Also vit will be seen that at the end of this period, or hangover time, the protection is abruptly removed andthe detector D again becomes operative. I

In order tomake the length of the hangover period independent of the amplitude of the incoming wave in the upper transmission path, a device is provided to fix the maximum charge which may be placed on the condenser C A desirable volume limiting circuit is formed by the two-electrode vacuum tube V andthe resistance R connected in the input circuit of detector D, as shown in Fig. 1.

In order that the condenser C may be quickly charged without-the applicationof excessively high voltages, the condenser C and the resistance R are included in the output circuit of the detector D to permit the operation of the detector on that art of its characteristic which is most favora le to such quick charging without the application of excessively high voltages to the plates of the detector.

The inclusion of the inductance L and the capacity C provides a tuned circuit for filtransmission channel.

V, becomes conducting and the con- It has been stated that the length of the I 'hang'over period is independent ot the ampli-.

tude of the waves in'the upper or incoming transmission path. This will be more clearly understood if consideration is given to Figs.

2 and 3 of the drawing. Considering in detail, first, the diagram of Fig. 2, time is plotted against negative voltage. The curves E0, and Erindicate the voltages which control the hangover of the detector D The curve E0 corresponds to the charge on the condenser G which controls the biasing voltage on the grid of tube D The curve W represents the wave in the upper The mixed line indicates the normal biasing voltage placed on the grid of V, by the battery B. By means of the broken line E73,, there is represented the potential placed across the resistance 1%, upon the charging of the condenser C The wave in the transmission path, represented by W, causes the charge on the condenser C,- to be proportional to the amplitude of N and the charge on C to reach the maximum value. These charges hold up until the-wave ceases. Then, it is noted, the voltage Ea", drops oil gradually on the usual. condenserresistance discharge curve. The voltage E0 however, has an almost imperceptible drop, since the resistance R, is very large, or may be made to have any desired drop. As long as the voltage on thegrid of V is greater than the normal biasing voltage thereon, the voltage maintained on the rid of D by the charge on the condenser 1 holds up. As soon as the voltage on the grid of V reaches the value at which the filament plate circuit of that tube becomes conducting, then the protection voltage on the grid of D is abruptly removed, as is indicated by the curve B0,, condenser C being discharged through the tube V,. In other words, with reference to Fig. 2, from X to Y, the charge on 6,, represented by the curve E0 is raised to the maximum and held there in response to the wave in the transmission path, represented by W; from Y to Z, .this charge drops ofi only slightl the grade of the drop being steeper as t e value of the resistance R is removed from infinity; and at Z, the charge is abruptly reduced to zero when the charge on C represented by Er reaches the value of zero, and the voltage on the grid of V reaches the normal'biasing voltage from the battery B. The effective hangover time is that represented between Y and Z.

An examination of Fig. 3, in connection with Fig. 2, will show clearly that the length of the effective hangover period is quite independent of the amplitude of the wave in the incoming transmission path. The wave W of Fig. 3 is much smaller in amplitude than Nil the wave of Fig. 2; yet, the values of C and R giving the broken curve in Fig. 3 similar to the curve El of Fig. 2, the hangover in the case of Fig.3 is of the same length as in the case of Fig. 2, the maintaining charge on the condenser C being abruptly removed when the potential on the grid of V is reduced to the limit fixed bv the normal biasing voltage from battery B.

lVhilc the invention has been disclosed in one specific embodiment, which is deemed desirable. it is to be understood thatit ma be embodied in many other and widely di ferent forms within the scope of the appended claims.

lVhat is claimed is:

1. In a two-way telephone system including acircuit having a first path adapted for transmission in one direction and a second path adapted for transmission in the opposite direction, the first path being normally blocked and the second path being normally cleared, a two-wire line connected to said circuit at one end thereof, switching means for simultaneously clearing said first path and blocking said second path, and a threcelectrode vacuum tube detector associated with both of said paths for controlling said switching means; the method of preventing the operation of said switching means by echoes of waves in said second path which are reflected from the two-wire line over said first path, which consists in impressing on the grid of the detector in response to waves in the second path a negative voltage sutlicicnt to prevent operation of the detector by the echoes, simultaneously storing a quantity of energy to maintain said voltage on said grid, abruptly discharging said maintaining energy at. the termination of a certain period of time, and causing the length of said period to be determined independently of the amplitude of the waves in the second path.

2. In a two-way telephone system including a circuit having a first path adapted for transmission in one direction and a second path adapted for transmission in the opposite direction, the first path being normally blocked and the second path being normally cleared. a two-wire line connected to said circult at one end thereof, switching means for simultaneously clearing said first path and blocking said second path, and a three-electrode vacuum tube detector associated with both of said paths or controlling said switching means; the method of preventing the operation at said switching means by echoes of waves in said second path which are rctlected from the. two-wire line over said first path, which consists in impressing 0n the grid of the detector in response to waves in the second path a negative voltage directly proportional to the strength of transmission over said path, simultaneously storing a quantity of energy to maintain said voltage on said grid, abruptly discharging said maintaining energy at'the termination of a certain period of time, and causing the length of said period to be determined independently of the amplitude of the waves in the second path.

3. In a two-way telephone system including a circuit having a first path adapted for transmission in one direction and a second path adapted for transmission in the opposite direction. the first path being normally blocked and the second path being normally cleared, and a two-wire line connected to said circuit at one end thereof; a three-electrode racuum tube detector associated with both of said paths, switching means controlled there by for clearing the first path and simultaneously blocking the second patlnmeans for operating said detector in response to voice waves originating in the two-wire line and passing over the first path, means for impressing on the grid of the detector in response to waves in the second path a negative voltage sutlicicnt to prevent operation of the detector by echoes of said waves in the first path. means For maintaining said voltage on said grid tor a period of time independent. ot. the amplitude of the waves in the second path. and means for abruptly removing said voltage at the termination of said period.

4%. In a tub-way telephone system includl ing a circuit having a first path adapted for transmission in one direction and a second path adapted for transmission in the opposite direction, the first path being norn'ially blocked and the second path being normally clcaredfarul a two-wire line coin'lcctcd to sa id circuit at one end thereof: a lhree-clcctrotlc vacuum tube detector a sociated with both of said paths. switching means controlled them by For clearing the first; patl and simultaneously blocking the second path. means for operating said detector in response to voice waves ou 'inating in the two-wire line and passing over the first path, means for impressing on the grid of the detector in response to waves in the second path a negative voltage directly proportional to the strength of transmission over the second path. means for maintaining said voltz'tgc on said grid for a period. of time independent of the amplitude ot the waves in the second path. and means for aln'uptly removing said voltage the tez lillftl'lOll of said period.

In a t;vo-way telephone system including a circuit having first path adapted For transn' ion in one direction and a Fccond path adapted for transmission in the opposite direction. the first path being normally blocked and the second path being normally cleared, and a two-wire line connected to said circuit at one end thereof: a three-electrode vacuum tube detector associated with both of said paths, switchim means controlled thereby for clearing the. first path and simultaneously blocking the second path, means for llt) operating said detector in response to voice waves originating in the two-wire line and passing over the first path, means for impressing on the grid of the detector in response to waves in the second path a negative voltage sufficient to prevent operation of the to the waves in the second path for impress ing a negative voltage on the grid of said vacuum tube to prevent conduction over the filament-plate circuit thereof and the consequent discharge of the stored energy, and means for gradually reducingthe negative voltage on said last-named grid to the point of conduction over said filament-plate circuit, the eriod of reduction having a length indepen ent of the amplitude of the waves in the second path.

6. In a two-way tele hone system including a circuit having a rst path adapted for transmission in one direction and a second path adapted for transmission in the opposite direction, the first path being normally blocked and the second path being normally cleared and a two-wire hne connected to said circuit at one end thereof; a three-electrode vacuum tube detector associated with both of said paths, switching means controlled thereby for clearing the first path and simultar neously blocking the second path, meansfor operating said detector in response to voice waves originating in the two-wire line and passing over the first path, means for impressing on the grid of the detector in response to waves in the second path anegative voltage directly proportional to the strengthof transmission over the second path, means for simultaneously storing a quantity of en-- ergy to maintain said voltage on said grid, a three-electrode vacuum tube having its filament-pl ate circuit connected across said storing means, means responsive to the waves in the second path for impressing a negative voltage on the grid of said vacuum tube to prevent conduction over the filament-plate circuit thereof and the consequent discharge of the stored energy, and means for gradually reducing the negative voltage on said lastnamed grid to the point of conduction over said filament-plate circuit, the period of reduction havin a length independent of the amplitude of the waves in the second path.

In testimony whereof, I have signed my name to this specification this 7th day of September 1927.

HAROLD C. SILENT. 

